Digital switching systems have been developed which provide non-blocking digital switching in which pulse code modulation is used to digitize information such as voice channels to permit controllable switching of information on the inputs to the outputs of the digital switch. For example a total of 32 voice channels may be pulse code modulated and time division multiplexed so that selected channels on the inputs of the digital switch are connected to different outputs of the digital switch. The switching required to produce the time division multiplexed switching of different channels between selected inputs and outputs is produced by control switching signals applied to the digital switches of the switching matrix.
In the field of telephony numerous manufacturers make voice digitizing boards which convert individual channels (conversation) into a time division multiplexed PCM encoded format in which multiple bits encode respective samples of each channel. On a wire pair typically 24 or 32 different channels are encoded by assigning individual channels representative of a conversation to a particular time slot within a frame of the time division multiplexed PCM encoded information. Switching of the PCM information on a bus is controlled by a digital switch which permits the selective connection of individual channels through the public switched telephone network to particular telephone lines.
The voice digitizing products of manufacturers such as Dialogic Corporation, Natural Microsystems Corporation, Rhetorex and the assignee of the present invention, American Tel-A-Systems, Inc. have incompatible bus protocols which prevent direct interconnecting of buses of systems made by these manufacturers. The buses utilized by the aforementioned manufacturers and others have several possible clock rates such as 1.544 MHz, 2.048 MHz, 4.096 MHz and different 8 kHz synchronization of the frame rate as provided respectively by specifications of integrated circuits manufactured by National Semiconductor or Mitel Corporation and further synchronization signals which define superframes utilized for T1 service which are a combination of multiples of 6 basic frames at the 8 kHz frame rate. As a result of the diversity of the clock rates, synchronization and other timing signals utilized by the various manufacturers there is no compatible bus protocol for interconnecting information systems which have been made by the interconnection of boards provided by a particular manufacturer with an information system made by interconnecting boards from another manufacturer. As a result a user of a system having digitized voice to date is required to make difficult choices as to which manufacturers' products to purchase which limit the overall functional capability of a system and the ability to modify the system in the future as a consequence of the inability to interconnect subsystems formed from products manufactured by different manufacturers.
Circuit boards are marketed which may be inserted into the expansion slot of a standard PC such as a PC-AT to control different types of analog telephone ports. These boards provide analog telephone line port receptacles for connection to the public switched telephone network (PSTN). The analog ports may be for a station port which supplies battery feed capable of applying ring voltage to a line along with normal line status and control functions for the purpose of interfacing two telephones; a loop start port of the so-called "plain old telephone service" which includes all normal line status monitoring; a ground start port which provides a ground start line interface; a DID port which provides all known RS-464 and IEEE standard DID protocols including wink, immediate start as well as DTMF and pulse dialing; an E & M port which may be configured for type I or II signalling and a battery feed port which provides battery but cannot provide ring voltage. These circuit boards have programmable jumpers or switches which permit each of the ports of the circuit to be programmed to have the desired type of line characteristic. The disadvantage of this type of circuit board is that all of the circuits necessary for supporting the types of possible line interfaces which may be provided must be present on the board even though the customer may only want to use a fraction of the overall number of line type interfaces that are supported by the board. As a result, the expense of the circuit board is increased and further the number of analog lines which may be connected to the board is lessened as a consequence of the necessary electronics for supporting all of the lines taking up space which lessens the number of ports which may be on the board.
Another design of a circuit board for interfacing a PC with analog lines does not permit the programming of the characteristic of the line to which the circuit board is to be connected. Each circuit board of this type is hard-wired to support one or more types of analog line interfaces. As a result, this product is not programmable and may be only used for specific applications for specific connections to analog lines which prevents it from being reconfigured by the customers to interface with different types of analog lines.